1. Field of the Invention
This invention concerns phosphonitrile oligomers containing maleimidophenoxy group and their intermediate products as starting material for polymers excellent in heat resistance and fire resistance, as well as concerning a novel process for producing these compounds.
2. Description of the Prior Art
Phosphonitrile oligomers obtained by reacting a phosphonitrile chloride oligomer having a cyclic or linear structure ##STR1## where n represents an integer of 3 or greater, particularly, a cyclic phosphonitrile chloride oligomer wherein n is 3 or 4, namely, ##STR2## or a mixture of phosphonitrile chloride oligomers containing them as the main ingredient with an organic hydroxy compound, or polymers obtained through the polymerization of them are excellent in heat-resistance, cold-resistance, lubricity, electrical insulation or chemical stability. Accordingly, application uses for these compounds have been developed generally as various kinds of industrially materials.
It is said that, among these phosphonitrile oligomers, those polymers obtained by the thermal polymerization of hexakis(4-maleimidophenoxy)cyclotriphosphazene represented by the formula ##STR3## where A represents ##STR4## or a tris(4-maleimidophenoxy)-tris(phenoxy)cyclotriphosphazene represented by the formula ##STR5## where T represents ##STR6## are extremely excellent in heat resistance and fire resistance and it is said that carbonaceous cloth laminated therewith are not burnt at all even heated to 300 .degree. C. in gaseous oxygen.
These compounds are described in U.S. Pat. No. 4,550,177, or "Journal of Polymer Science" : Polymer Chemistry Edition, vol. 22, p927-943 (1984), etc. According to the above mentioned literatures, the compound (1) is prepared in accordance with the following scheme ##STR7## where E represents ##STR8## where G represents ##STR9## where L represents ##STR10## where M represents ##STR11##
Furthermore, the compound (II) is prepared by similar steps excepting for replacing one-half amount of the nitrophenoxy group bonded in the first reaction step of the above-mentioned schemes with the phenoxy group.
Thus, the method described in these literatures is extremely complex including a plurality of steps, that is, a reaction between a trimer of cyclic phosphonitrile chloride and 4-nitrophenolate or a reaction between 4-nitrophenolate and phenolate, followed by hydrogen reduction of the nitro group under an elevated pressure, maleimidation of the resultant amino group and preparation of the aimed product through the ring closure by dehydration and, since the final yield is low, it can not be practiced from an economical point of view.